An understanding of how the nervous system processes visual information will depend upon a complete description of both the physiological and structural properties of particular visual areas. Such description requires knowledge of the way in which the visual field is represented in an area, the receptive field characteristics of its cells, the afferents which innervate an area, and the efferent connections of its cells. Once these gross features of organization are known, it is possible to begin analysis of the detailed circuitry of a visual structure. A circuit analysis requires identification of specific cell types, the type, density and spatial distribution of their inputs, and how they are interconnected. The cat superior colliculus is an excellent choice for studying detailed circuitry because its topography, receptive field features, and the laminar distribution of many of its inputs are well established. On the other hand, very little is known about the specific cell types or the classes of synapses which each cell type receives. This proposal is directed at a detailed description of cell type within the superficial gray layer of the colliculus. Particulate cell types will be identified on the basis of projection site, cell volume and shape, dendritic arborization, and the patterns of synaptic input to each cell. Identification of projection site will be determined by retrograde labelling with horseradish peroxidase. The ipsilateral and contralateral retinal afferents and area 17 and 18 cortical terminals will be examined by labelling with tritiated amino acids using electron microscope autoradiography. The detailed synaptology of identified cells will be determined using electron microscope serial section reconstruction. The results will produce an accurate classification of cell type in the superficial gray and will determine whether functionally distinct W and Y retinal afferents can be distinguished morphologically and whether they contact different classes of cell. This analysis will established a structural foundation for delineating the physiological operations which occur in this important midbrain structure.